The present invention relates to a power amplifier used at mobile communication base stations primarily of cellular phones, etc.
In recent years, for transmitters of base stations of mobile communication apparatus, power amplifiers with high-efficiency and as linear characteristics as possible are required for batch-amplifying a large number of signal channels. In order to make the power amplifier characteristics as linear as possible, it is essential to adopt distortion compensation using, for example, feed-forward method, etc. as well as to reduce distortion of power amplifiers themselves.
FIG. 31 is a drawing showing a conventional power amplifier 600. The power amplifier 600 includes an input terminal 601, output terminal 602, baluns 603, 604, and amplifiers 605, 606. Amplifiers 605, 606 include amplifying elements 611,612, matching circuits 613, 614 on the input side, matching circuits 615, 616 on the output side, capacitors for input-side bias circuit 617, 618, and capacitors for output side bias circuit 619, 620. Amplifiers 605 and 606 operate as push-pull amplifiers. Each of capacitor 619 and 620 actually comprises a plurality of capacitors. That is, capacitors that provide low impedance at the specified signal frequency and capacitors that provide low impedance at the frequency that corresponds to the frequency difference (frequency intervals) when a plurality of signals are entered. By mounting capacitors for output-side bias circuit 619, 620, intermodulation distortion generated in amplifiers is suppressed and characteristics of power amplifiers can be brought to linearity as much as possible.
However, in the conventional power amplifier 600, capacitors 619, 620 must be mounted for bias circuits of each of the push-pull amplifiers 605 and 606, and the number of parts is increased. In addition, because capacitors 619, 620 must have, in general, a large capacity and low resistance, the increased number of parts increase the overall circuit scale. Furthermore, fluctuations are generated in the degree of suppression of intermodulation distortion in accord with the frequency intervals of a plurality of signals entered in amplifiers.
It is an object of the present invention to provide a power amplifier that reduces intermodulation distortion generated in amplifiers while reducing the number of parts.
A power amplifier according to the present invention comprises a first balun, to which a combined signal combining two signals of different frequencies is inputted, and which outputs, based on the combined signal, the first and the second signal whose phase are opposite; a first amplifier which outputs the first amplified signal containing the frequency component comprising a difference of two signal frequencies from the first signal, the second amplifier which outputs the second amplified signal containing the frequency component comprising a difference of two signal frequencies from the second signal, and the second balun which outputs the combined signal of the first and the second amplified signals, wherein the frequency component contained in the second amplified signal is entered into the interval from the first balun to the second balun via the first amplifier to reduce the frequency component, and the frequency component contained in the first amplified signal is entered in the interval from the first balun to the second balun via the second amplifier to reduce the frequency component.
According to the present invention, when a plurality of signals are entered in the push-pull amplifier, the component of the frequency interval xcex94f generated at the output terminal in the push-pull amplifier is reduced. Consequently, the intermodulation distortion generated at the amplifier can be suppressed and no capacitor is necessary to be disposed in the bias circuit as is the case of a conventional amplifier, the number of parts can be reduced. As a result, the linearity of the amplifier is increased, and a power amplifier that can amplify a large number of signal channels collectively without distortion can be obtained.